The present invention relates to a device for driving a rotary body and, more particularly, to a device for driving a second rotary body which is movable toward and away from a first first rotary body that is mounted on a machine body by transmitting a rotating force from a drive source to one end of the second rotary body, while maintaining the second rotary body in pressing contact with the first rotary body in a uniform pressure distribution. Both of the first and second rotary members to which the present invention is applicable may have an elongate cylindrical or columnar configuration.
A driving device of the type described is incorporated in various kinds of machines. With an electrophotographic copier, printer, facsimile machine or similar image forming apparatus, for example, it is a common practice to hold an image carrier in the form or a drum, or first rotary body, in pressing contact with a developing roller, or second rotary body. The above-described type of driving device drives the developing roller in a rotary motion. In this kind of application, it is often required that the first and second rotary bodies be pressed against each other in a uniform pressure distribution along their axes. Specifically, an image forming apparatus has a developing roller (second rotary body) and an image carrying drum (first rotary body) which are pressed against each other. While the roller and drum are individually rotated, a latent image electrostatically formed on the drum is developed by a toner and the resulting toner image is transferred to a paper sheet. This kind of development is generally referred to as contact development. To insure stable and high-quality development of the latent image, it is preferable that the roller and drum be pressed against each other by a constant pressure which is not excessively high or excessively low. This may be implemented on the assembly line by positioning the roller and drum accurately parallel to each other and mounting them on the body of the apparatus rotatably but unmovably otherwise.
In general, however, the developing roller and drum have peripheral surfaces which are somewhat eccentric relative to their axes of rotation or not accurately parallel to the latter or even undulated, due to errors and other causes particular to the production line. Hence, the parallelism of the roller and drum achievable in practice is limited. It follows that maintaining the roller and drum in contact in a uniform pressure distribution which is not excessively high or excessively low along their axes is extremely difficult. One possible approach for eliminating this difficulty is to mount the roller in such a manner as to be movable toward and away from the drum which is unmovable, and to urge the roller against the drum by biasing means which may comprise springs, as proposed in the art. This scheme is successful in insuring a uniform pressure distribution between the roller and the drum despite the previously stated defects particular to the peripheral surfaces of the roller and drum.
Usually, the developing roller is driven by a device which is composed of a gear or similar transmitting member mounted on one end of the roller, a drive gear securely supported by the machine body and meshed with the transmitting member, and a drive motor mounted on the machine body for driving the drive gear. However, when a driving force is applied to only one end of the roller and the developing roller is movably supported as previously stated, a force which is deviated by the pressure angle from the common tangent of the two gears, i.e., a so-called tangential force acts on the driven end of the roller only. In this condition, the pressure which the roller exerts on the drum differs from one end where the tangential force acts to the other end, tending to disturb the uniform contact of the roller and drum in the axial direction. This problem may be eliminated by elaborating the position of the gear mounted on the developing roller or by using an Oldham's coupling, as disclosed in Japanese Utility Model Publication No. 59-26373 by way of example. Such a scheme, however, often fails to achieve the expected advantage when the torque of the developing roller is large or when individual components have scattering as to the position due to errors introduced in the assembly of the gears and coupling.
The undesirable occurrence discussed above is observed with various kinds of machines or various portions when, for example, the second rotary body in the form of a charging roller or similar roller is pressed against the first rotary body in the form of an image carrying drum or when the first rotary body of an offset printer which is an offset roller is pressed against the second rotary body in the form of an ink roller. This is also true with an arrangement wherein spacer rollers mounted on the second rotary body are pressed against the first rotary body and an arrangement wherein another form of torque transmission device is used.